title: Calcium-dependent activator protein for secretion 1 (CAPS1) binds to syntaxin-1 in a distinct mode from Munc13-1.
abstract: Calcium-dependent activator protein for secretion 1 (CAPS1) is a multidomain protein containing a Munc13 homology domain 1 (MHD1). Although CAPS1 and Munc13-1 play crucial roles in the priming stage of secretion, their functions are non-redundant. Similar to Munc13-1, CAPS1 binds to syntaxin-1, a key t-SNARE protein in neurosecretion. However, whether CAPS1 interacts with syntaxin-1 in a similar mode to Munc13-1 remains unclear. Here, using yeast two-hybrid assays followed by biochemical binding experiments, we show that the region in CAPS1 consisting of the C-terminal half of the MHD1 with the corresponding C-terminal region can bind to syntaxin-1. Importantly, the binding mode of CAPS1 to syntaxin-1 is distinct from that of Munc13-1; CAPS1 binds to the full-length of cytoplasmic syntaxin-1 with preference to its "open" conformation, whereas Munc13-1 binds to the first 80 N-terminal residues of syntaxin-1. Unexpectedly, the majority of the MHD1 of CAPS1 is dispensable, whereas the C-terminal 69 residues are crucial for the binding to syntaxin-1. Functionally, a C-terminal truncation of 69 or 134 residues in CAPS1 abolishes its ability to reconstitute secretion in permeabilized PC12 cells. Our results reveal a novel mode of binding between CAPS1 and syntaxin-1, which play a crucial role in neurosecretion. We suggest that the distinct binding modes between CAPS1 and Munc13-1 can account for their non-redundant functions in neurosecretion. We also propose that the preferential binding of CAPS1 to open syntaxin-1 can contribute to the stabilization of the open state of syntaxin-1 during its transition from "closed" state to the SNARE complex formation.
authors.affiliation: Division of Fundamental Neurobiology, University Health Network, Toronto, Ontario M5T 2S8, Canada.
authors.forename: Leon
authors.initials: L
authors.lastname: Parsaud
authors.forename: Lijun
authors.initials: L
authors.lastname: Li
authors.forename: Chang Hun
authors.initials: CH
authors.lastname: Jung
authors.forename: Seungmee
authors.initials: S
authors.lastname: Park
authors.forename: Ner Mu Nar
authors.initials: NM
authors.lastname: Saw
authors.forename: Sanghyun
authors.initials: S
authors.lastname: Park
authors.forename: Moo Yup
authors.initials: MY
authors.lastname: Kim
authors.forename: Shuzo
authors.initials: S
authors.lastname: Sugita
mesh_terms: D000818:Animals; D002135:Calcium-Binding Proteins / Q000235:genetics / Q000378:metabolism*; D057809:HEK293 Cells; D006801:Humans; D051379:Mice; D009419:Nerve Tissue Proteins / Q000235:genetics / Q000378:metabolism*; D009489:Neurosecretion / Q000502:physiology*; D016716:PC12 Cells; D010449:Peptide Mapping; D011485:Protein Binding / Q000502:physiology; D055550:Protein Stability; D017434:Protein Structure, Tertiary; D051381:Rats; D050600:SNARE Proteins / Q000235:genetics / Q000378:metabolism; D012441:Saccharomyces cerevisiae / Q000235:genetics; D050827:Syntaxin 1 / Q000235:genetics / Q000378:metabolism*; D020798:Two-Hybrid System Techniques
chemical_list: C059605:CAPS protein, human; C510820:Cadps protein, mouse; C485812:Cadps protein, rat; D002135:Calcium-Binding Proteins; D009419:Nerve Tissue Proteins; D050600:SNARE Proteins; D050827:Syntaxin 1; C485548:UNC13B protein, human; C503321:Unc13a protein, mouse; C103912:Unc13a protein, rat
keywords: Exocytosis; Membrane Fusion; Secretion; Snare Proteins; Vesicles